Projection optics for EUV lithography generally include a plurality of reflective optical elements, that is to say mirrors, which are held in position relative to one another with a precision in the nanometer range or even below. In order to meet these high demands on precision and stability, and in order to provide possibilities to undertake fine optical setting to compensate long-term effects, one or more of the optical elements can be actuated and/or manipulated in up to six degrees of freedom. The optical elements or the mirrors and/or their associated actuators are held in the desired position in this case via a structure which is also termed the “lens barrel”. Such a structure is disclosed, for example, in U.S. Pat. No. 6,750,949. In the document mentioned, both the actuators and the associated sensors are arranged on the same structure, the result of this being that a sufficient dynamic and thermal stability of the structure is ensured for the optics to function without trouble. It is usually desired to this end for the first natural frequency (or first eigenfrequency) of such a structure to be higher than the control bandwidth of the actuators, in particular higher by a factor of five. The thermal stability is selected such that under all conceivable conditions of use the image position is not displaced by more than one nm, displacements of below 0.5 nm between calibrations of the image position being better.
One alternative possibility is described in U.S. Pat. No. 6,864,988, in the case of which the abovementioned structure is split into two parts, a so-called force frame which absorbs static or dynamic loads, and a measurement structure which is also denoted as a metroframe or sensor frame and serves as a reference for a multiplicity of sensors with the aid of which the mirror position is measured. In this embodiment, the high dynamic and thermal demands apply only to the measurement structure and not to the force frame. In this case, the measurement structure is decoupled from the force frame both with respect to vibrations and to thermal influences such that the measurement structure determines the stability and/or the resistance of the projection optics to disturbance.
A third possibility is set forth in U.S. Pat. No. 7,221,460, in which one or more measurement structures are kinematically connected to the force frame. It is true that this variant does not have the theoretically ideal isolation properties of the abovementioned embodiment, but it is substantially easier to produce mechanically.
Moreover, U.S. Pat. No. 6,549,270, U.S. Pat. No. 6,593,585 and JP 2004-327807 describe various configurations of the design of actuators and sensors of such EUV projection optics.
It is common to all the abovementioned embodiments that the optical elements and, in particular, the surfaces of the optical elements are arranged in a common space with the sensors and actuators of the device, the result being that the service life of the optics can be substantially shortened because of contaminations which originate from the elements.